Synthesis and photoinduced behavior of DPP-anchored nitronyl nitroxides: a multifaceted approach

Understanding and controlling spin dynamics in organic dyes is of significant scientific and technological interest. The investigation of 2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione derivatives (DPPs), one of the most widely used dyes in many fields, has so far been limited to closed-shell molecules. We present a comprehensive joint experimental and computational study of DPP derivatives covalently linked to two nitronyl nitroxide radicals (DPPTh-NN2). Synthesis, single crystal X-ray diffraction study, photophysical properties, magnetic properties established using steady-state and pulse EPR, fast spin dynamics, and computational modelling using density functional theory and ab initio methods of electronic structure and spectroscopic properties of DPPTh-NN2 are presented. The single-crystal X-ray diffraction analysis of DPPTh-NN2 and computational modeling of its electronic structure suggest that effective conjugation along the backbone leads to noticeable spin-polarization transfer. Calculations using ab initio methods predict a weak exchange interaction of radical centers through a singlet ground state of DPPTh with a small singlet–triplet splitting (ΔEST) of about 25 cm−1 (∼0.07 kcal mol−1). In turn, a strong ferromagnetic exchange interaction between the triplet state of DPPTh chromophore and nitronyl nitroxides (with J ∼ 250 cm−1) was predicted.


Introduction
Discovered in the early 1970s, 1 diketopyrrolopyrroles (2,5-dihydropyrrolo [4,3-c]pyrrolo-1,4-diones, DPPs) remain one of the most widely used dyes, nding application in many areas of high technology.Numerous studies have been devoted to the development of DPP derivatives as high-performance materials for use in various applications such as high-performance pigments, organic eld-effect transistors, bulk-heterojunction solar cells, dye-sensitized solar cells, organic light-emitting diodes, uorescence imaging, and many other elds. 2 Despite the signicant progress achieved by researchers in the exploration of DPP-based compounds, only recently have their conjugated derivatives substituted with stable radical groups been obtained.Single crystal X-ray data on a DPP derivative bearing radical groups, namely, bis(thiophen-2-yl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione with terminal nitronyl nitroxide groups (DPP Th -NN 2 ) were rst reported in 2020. 3ater, nitronyl-nitroxide diradicals with extended DPPcontaining linkers (DPP Th -Ph-NN 2 , DPP Fu -Ph-NN 2 , and DPP Ph -Ph-NN 2 , Fig. 1) were obtained and partially characterized. 4 In addition to the listed diradicals (Fig. 1), perylene bisimide (PBI) and iso-indigo (IIn) conjugated diradicals were also synthesized. 4Although all diradicals retain the intrinsic optical properties of the dyes or, more specically, pi-conjugated chromophore, at the same time they exhibit an indirect spin coupling between two distant paramagnetic centres.Room temperature EPR data 4 provided only nearly identical 9-line spectra, demonstrating that the exchange coupling parameter J is much larger than twice the hyperne coupling constant.The distances between the radical units were very large (C2-C2 0 distance in the range 2.2-2.4 nm for DPP-based diradicals) due to the additional phenyl rings connecting the NN moieties to the DPP core. 4 To date, comprehensive temperature-dependent studies of liquid and frozen solutions via EPR spectroscopy, as well as magnetization measurements of polycrystalline powder, have not been conducted.These analyses are crucial for a deeper understanding and clear differentiation of the magnetic properties of such diradicals.Earlier, Matsuda et al. demonstrated the exponential distance dependence of the exchange interaction parameter over extended phenylacetylene bridged NN biradicals. 5Their ndings indicate that the EPR spectra of the examined diradicals in liquid phase will display a consistent nine-line pattern (indicative of strong exchange) when the distance between the C2 atoms of the NN moieties ranges from 0.7 to 2.7 nm.This observation is coupled with signicant variations (spanning orders of magnitude) in the exchange coupling parameters, highlighting the sensitivity of magnetic interactions to molecular spacing within these systems.
Thus, the magnetic properties of DPP-based diradicals require much more in-depth studies.Since DPP derivatives have found applications in the solar cells, OLEDs, uorescence imaging, etc., 2d-g information available about their electronic absorption spectra is insufficient to understand their excitation relaxation dynamics which is very important to broaden the scope of these molecules in respective elds.Our study was undertaken with the aim of quantitatively elucidating the magnetic properties of the DPP Th -NN 2 diradical.Continuouswave and time-resolved EPR spectra were recorded, and exchange and dipole-dipole interactions were theoretically assessed not only in the ground state of DPP linker, but also in the case of its triplet excitation.Furthermore, we tried to resolve the issue of possible relaxation pathways for the excitation of the DPP chromophore.To do this, we performed both DFT and high-level calculations of the electronic structure of the DPP Th -NN 2 diradical.In this paper, we described in detail the synthesis of DPP Th -NN 2 , single crystal analysis, insights into electronic structure, and the results on the continuous wave, echodetected and time-resolved EPR study.

Results and discussion
Synthesis and structure of the DPP Th -NN 2 diradical The synthetic route to the DPP Th -NN 2 diradical is shown in Scheme 1.The diradical precursor, the corresponding dibromo-substituted DPP Th -Br 2 , was synthesized according to the literature-reported procedure 6 with necessary modications.To introduce two nitronyl nitroxide (NN) groups into the DPP Th core, we chose a palladium catalysed cross-coupling of bromoderivative of DDP with the (nitronyl nitroxide-2ide)(triphenylphosphine)gold complex.1][12] Thus, a new path has been opened to various in-demand nitronyl nitroxides for a variety of research elds, including radical chemistry, the creation of high-spin systems and rechargeable batteries, the design of molecule-based magnets and molecular units for spintronics. 13,14Naturally, using the described approach, the DPP Th -NN 2 diradical was successfully obtained in a moderate yield of ∼72% when using Pd(PPh 3 ) 4 catalyst (Scheme 1).
In chemistry of diradicals, X-ray diffraction analysis is of paramount importance because it unveils valuable information about molecular organization and interactions in the solid state.We succeeded in growing single crystals of DPP Th -NN 2 by slow diffusion of methanol into its solution in CH 2 Cl 2 at 5 °C.The crystalline product appeared to be stable at ambient conditions and no phase transitions were observed in the DSC analysis in the temperature range from 20 °C to the melting point of 216 °C (see ESI †).
At room temperature, attempts to perform an X-ray diffraction analysis of DPP Th -NN 2 single-crystal samples failed.Cooling the single-crystal sample at 193 K led to the freezing of the alkyl-chain movement, which made it possible to solve the molecular and crystal structure of the DPP Th -NN 2 diradical.According to XRD, the crystal structure of DPP Th -NN 2 is centrosymmetric (space group P 1) and contains two crystallographically independent molecules in an asymmetric unit (hereaer called as A and B).The molecular structures of A and B DPP Th -NN 2 molecules are depicted in Fig. 2. Bond lengths and angles in both forms of the molecule are similar (Table S1 †) and comparable to typical values for similar molecular fragments, as indicated by a search in the Cambridge Structural Database using the Mogul program. 15n the DPP Th -NN 2 diradical, the p-conjugated system, consisting of the DPP core, thiophene rings, and nitronyl nitroxide moieties, presents high planarity with small interplanar twist angles.In molecules A and B, dihedral torsion angles between planes of the nitronyl nitroxide groups and planes of the nearest side thiophene rings are 2.90 and 13.24°respectively.In turn, the two thiophene rings are imposed in anti-orientation with respect to each other and twisted by 9.16°(in molecule A) and 20.69°(in molecule B) relative to the mean plane of DPP Th .The observed planarity of the thiophene-nitronyl nitroxide system leads to short intramolecular H-bonds O/H-C (2.41 and 2.45 Å, dotted lines in Fig. 2), as well as extremely short contacts between O NO and S atoms (2.74 and 2.79 Å) compared to the sum of the van der Waals radii of the S and O atoms (3.32 Å, dashed purple lines in Fig. 2).Small torsion angles provide efficient conjugation along the backbone, thereby facilitating the noticeable transfer of spin-polarization from the NN radical units to the DPP Th chromophore, as observed in the EPR spectra and demonstrated by DFT calculations (Fig. S6, ESI †).
The crystal packing shows that the molecules adopt weak intermolecular p-p stacking separated by distance of 3.74 Å between centroids of the DPP Th moieties (Fig. 3).Inside such stacks, short intermolecular contacts C-H/O NO (2.36, 2.45, 2.56, and 2.64 Å) are realized between the oxygen atoms of the nitroxide fragments and the hydrogen atoms of the methyl groups (Fig. 4a).Similar contacts are oen observed in crystals of nitronyl nitroxides, and they are characterized by a rather high binding energy. 16Because of this, interactions of this type have a signicant effect on the motif of the crystal packing of nitroxide radicals.The most important consequence of C-H/ O NO interactions is the geometry of the mutual arrangement of nitroxide groups and, above all, the distances between the oxygen atoms of paramagnetic centers, since the latter  predetermine the magnetic properties of paramagnetic samples.Moreover, C-H/O NO interactions predetermine the relative rotation angle of molecules in the stacks, as well as the shortest intermolecular distances between nitroxide oxygen atoms (3.617 and 3.887 Å).In addition, one can see multiple short intermolecular C-H/O NO contacts (2.40, 2.55, and 2.64 Å) between the DPP Th -NN 2 diradicals belonging to neighbouring stacks (Fig. 4b).These interactions lead to the shortest intermolecular distances between the nitroxide oxygen atoms, equal to 3.653 Å.On the whole, in the solid phase of DPP Th -NN 2 , the shortest O/O distances between O atoms of nitroxide groups exceed 3.617 Å, which is much greater than the sum of the van der Waals radii of O atoms (3.04 Å).Therefore, in the crystalline phase of DPP Th -NN 2 diradicals, both intra-and intermolecular exchange interactions should be weak.
DFT and ab initio calculations of the electronic structure and magnetic properties of the DPP Th -NN 2 diradical To better understand the magnetic and spectroscopic properties of the DPP Th -NN 2 diradical, we performed a series of DFT and ab initio calculations.All calculations were performed for model geometries that differ from the geometry of the DPP Th -NN 2 by replacing the long n-hexyl substituents with methyl groups.In addition, two types of model geometry were used in the calculations: one was based on the XRD analysis, and the other was optimized in a toluene solution.
Calculations of intra-and intermolecular exchange interactions.Since the DPP Th -NN 2 diradical has SOMOs of a disjoint type (Fig. S7, S8, ESI and discussion ibid †), the parameters of the intramolecular exchange interaction between the NN fragments were calculated at the SA-CASSCF/NEVPT2 level of theory. 14The largest active space used in these calculations consisted of 14 electrons on 13 MOs (Fig. S8, ESI †).For the model geometry based on XRD analysis, the parameter J was calculated to be −11.3cm −1 (for molecule A) at the highest level of calculations.As expected, the BS-DFT calculations overestimated absolute value of J (−60, −53 and −79 cm −1 with B3LYP, M06 and M06-2X functionals, respectively).Thus, the intramolecular exchange interaction in the diradical is indeed very weak and antiferromagnetic, and the ground state of the diradical is diamagnetic.We also calculated the parameters of intermolecular exchange interactions of neighboring radical fragments in the stack.Interestingly, for a pair of diradicals, J parameters of different sign (−5.6 and 6.4 cm −1 ) were predicted for two pairs of adjacent radical fragments (Fig. S9, ESI †).An antiferromagnetic exchange interaction was predicted in the case when ONCNO fragments of neighboring radical fragments are almost parallel, and a ferromagnetic one, in the case of a signicant deviation from parallel arrangement of the corresponding fragments.
Electronic absorption spectra of DPP Th and DPP Th -NN 2 .Before proceeding to the calculation and analysis of exchange interactions in the excited states of the DPP Th -NN 2 diradical, it is reasonable to interpret and analyze its electronic absorption spectrum and compare it with the spectrum of the parent DPP Th .Fig. 5 displays the electronic absorption spectra of both the DPP Th and DPP Th -NN 2 in toluene solution at room temperature.The DPP Th shows an intense absorption band in the visible region with two maxima; this pattern is characteristic of diaryl-substituted DPP core.2f,g According to the TD-B3LYP calculation, this structured band corresponds to a single electronic excitation (507 nm, f = 0.48, Fig. 5), in which the electron promotion from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) dominates (Fig. 6a).The next transition with a much smaller intensity is predicted at 360 nm (f = 0.0034).Thus, the maxima at 550 and 512 nm and the shoulder at about 480 nm represent the vibrational structure of the long wavelength band (with n ∼ 1350 cm −1 ).
The electronic absorption spectrum of the DPP Th -NN 2 diradical contains a very similar intense absorption band with the same structure as in the DPP Th spectrum, but noticeably red-shied.Note that the DPP Th -NN 2 spectrum was calculated for the high-spin triplet state of the diradical.The wavefunction of the ground open-shell singlet cannot be correctly described in the one-determinant approximation, and thus its UV-Vis spectrum cannot be calculated at the TD-DFT level.Fig. S7 (ESI †) represents the MO diagram with a series of aand b-type MOs involved in electronic excitations of the DPP Th -NN 2 diradical.It can be seen that highest occupied and lowest unoccupied MOs of both aand b-types are similar to the HOMO and LUMO of the DPP Th (Fig. 6a) and differ from them only in some delocalization to NN fragments.According to the TD-UB3LYP calculation, the intense structured band in the spectrum of diradical also corresponds to a single intense transition (563 nm, f = 0.96, Fig. 5), which in turn corresponds to the HOMO / LUMO promotions for aand b-electrons (Fig. 6b).Since these HOMO and LUMO are similar to those of the parent DPP Th (except for a slight delocalization), the intense long wavelength bands of the precursor and diradical have very similar shape and intensity, but the diradical band is red-shied.
In addition, the diradical spectrum shows a weak feature (around 680 nm) at the tail of the absorption band discussed above.According to the calculations, the DPP Th -NN 2 has two electronic transitions in this region: at 672 nm (f = 0.0037) and 673 nm (f = ∼10 −8 ).In turn, the MO diagram (Fig. S7, ESI †) shows that two SOMOs (MO-166a and MO-167a) localized on the NN fragments are practically degenerate and lie below the HOMO by about 0.2 eV.The corresponding unoccupied MO-168b and MO-169b localized on the NN fragments (partners of the a-SOMOs) are also practically degenerate and lie above LUMO by about 0.2 eV.It is these MOs, along with HOMO and LUMO, that are involved in long wavelength low intensity transitions.Both transitions consist mainly of two contributions: the promotion of an a-electron from the SOMO to the LUMO and the promotion of a b-electron from the HOMO to the SOMO partner (Fig. 6c).These transitions are more complex than those previously calculated for a number of nitronyl nitroxide radicals. 14nalysis of the spin-Hamiltonian parameters in the excited states.In the excited states of the DPP Th -NN 2 diradical, a much stronger exchange interaction is expected.example, the exchange interaction of the local excited triplet state of the DPP Th core with the doublet states of the NN fragments should lead to four excited states: a singlet, two triplets, and a quintet.Knowledge of the sequence and splitting of these multiplets, as well as zero-eld splitting of triplet and quintet states, is very important for the interpretation of time-resolved and echodetected EPR spectra, and the phase relaxation kinetics in the presence and absence of laser radiation.
The results of the high-level calculations for the six lowenergy DPP Th -NN 2 multiplets are presented in Fig. 7 and S10 (ESI †), as well as in Table 1.Calculations were carried out both for the optimized and XRD-based geometries.An analysis of the CASSCF wave functions demonstrates that the Q 1 state, as well as T2, T3, and S1 states, actually arose as a result of the exchange interaction of the local triplet state corresponding to the HOMO / LUMO excitation (Fig. 6a) with NN radical fragments (for details, see ESI, Section 3 †).The energy splitting between these multiplets corresponds to the exchange interaction of the local triplet state with doublets of radical fragments (with parameter J 1 ) and between radical fragments (with J 2 ), respectively (Table 1).Results of Table 1 show that the exchange interaction of the radical fragments is weak and antiferromagnetic for both the ground and triplet excited states.In turn, the exchange interaction between the local triplet state and NN fragment is strong and ferromagnetic.The latter can be explained by the McConnell I mechanism, 17 as carbon atom of ON-C-NO fragment has high negative spin population and is bound with carbon atom of thiophen ring with high positive spin population (Fig. S11 †).
For the parent DPP Th , previous femtosecond transient absorption spectroscopy and time-correlated single-photon counting studies 18 have demonstrated that the singlet excited S 1 state of this compound relaxes directly to the ground state via internal conversion and uorescence, bypassing the triplet state.For the DPP Th -NN 2 the energy diagram is much more complicated.Excitation of the diradical by the 532 nm laser Table 1 Results of the CASSCF(14,13)/NEVPT2/def2-TZVP calculations of the low-energy spectrum of the DPP Th -NN 2 , consisting two singlet, three triplet and one quintet states, as well as the parameters of corresponding exchange interactions a and the zero-field splitting (ZFS) parameters (D, E/D) Electronic state E, cm −1 and J, cm a Parameters J 1 and J 2 of the exchange interaction correspond to the spin Hamiltonian of the form pulse leads to both singlet and triplet states arising from the exchange interactions of NN fragments through the excited singlet state localized mainly on the DPP Th core.However, as was discussed previously, slightly below this pair of states, there are also states responsible for a weak feature in the UV-Vis spectrum (Fig. 5) (4 states in total -two triplets and two singlets).Finally, as shown by calculations, this sequence of excited states is closed by singlet, triplet, and quintet states presented in Fig. 7. Thus, in the case of the DPP Th -NN 2 diradical, between the state populated upon excitation and the ground state there are a large number of states, the energy gaps between which are small.In addition, among these states there are pairs of very close singlet and triplet levels that arise due to the exchange interaction of NN fragments.Therefore, one might expect a very fast conversion of the excited state (singlet or triplet), which occurs immediately aer the excitation, into the Q 1 state.At room temperature, the Q 1 state is expected to be short-lived due to thermally activated relaxation through the T 2 state.However, such a process is absent at cryogenic temperatures, and one can expect a sufficiently long-lived Q 1 state at these temperatures.
The steady-state and echo-detected EPR spectra of the DPP Th -NN 2 .Although the ground state of the DPP Th -NN 2 diradical is singlet and diamagnetic, its triplet state at room temperature is substantially populated (about 70% using predicted J value).Thus, the EPR spectrum of DPP Th -NN 2 in a toluene solution at ambient temperature (Fig. 8a) was recorded, and it is characteristic of bis(nitronyl-nitroxide) systems with an intramolecular exchange interaction signicantly exceeding the hyperne coupling (jJj [ A N ), which is also consistent with the results of our calculations (jJj = 3.7 × 10 3 mT, Table 1).The spectrum has a center at g iso = 2.0065 and contains nine lines due to the coupling of two unpaired electrons with four equivalent 14 N nuclei with jA N j/2 ∼ 0.374 mT, which is experimental evidence of the diradical nature of DPP Th -NN 2 .
The EPR spectrum of the DPP Th -NN 2 diradical in a frozen toluene solution at 135 K (Fig. 8b) is difficult to interpret, since the symmetry is partly lost and some anisotropic components may overlap in jDm S j = 1 region.This results in a different number of shoulders in the outermost regions of the spectrum.Therefore, the zero-eld splitting parameters can be estimated roughly from the low temperature EPR experiment.However, clear evidence of a diradical character comes from the observed jDm S j = 2 transition.This signal has a low signal-to-noise ratio (S/N), because the average value of the N/N, O/O and N/O distances is large and estimated at 15.3 Å (for both the XRD and optimized structures), which implies a small jDj and, accordingly, a low probability of this transition.In the point dipole approximation, a distance of 15.3 Å leads to jDj = 0.81 mT (7.6 × 10 −4 cm −1 ).Note that this value cannot explain the rather large width of the EPR spectrum (Fig. 8b).However, the calculation of the spin-spin contribution at the DFT level gave an even smaller but close value: D = −0.73mT (−6.8 × 10 −4 cm −1 ) and E/D = 0.014.Fig. 9a shows the echo-detected (ED) EPR spectra of the DPP Th -NN 2 diradical recorded at 10 K under the same conditions in the presence and absence of pulse laser excitation.The spectrum recorded in the absence of irradiation features one slightly asymmetric EPR line, the maximum of which corresponds to g = 2.008.This signal apparently corresponds to the lowest triplet state of the DPP Th -NN 2 diradical (T 1 ), and, according to calculations, its population at 10 K is low, although it cannot be ruled out that E(T 1 ) may be substantially overestimated.
In the presence of laser radiation, the observed EPR signal retains its shape, but becomes less intense.Based on the quantum chemical calculations and discussion presented above, one should expect that the lowest excited quintet state (Q 1 ) may be populated upon excitation, thus being paramagnetic.However, still we do not observe any reliable spectral manifestations of such state.In addition, the transverse relaxation curves in the presence and absence of photoexcitation are remarkably similar (Fig. 9b), indicating that they refer to the same paramagnetic species.
We also applied time-resolved (TR) EPR technique to probe the intermediate species formed upon photoexcitation of DPP Th -NN 2 in toluene glass at 80 K. TR EPR is based on continuous wave detection and typically is more sensitive to photoexcited paramagnetic species compared to pulse EPR.However, no TR EPR signals were observed.In addition, the sample with pure DPP Th moiety (without radicals attached) has been investigated with the same zero TR EPR signal.The failure to detect the TR EPR signal in the case of DPP Th is consistent with the relaxation of the S 1 state directly to the ground state, as discussed above.The failure in the case of the diradical may be due to the short lifetimes of T 2 , T 3 and Q 1 states, if they are actually formed.
The observed decrease of the echo signal under light can be assigned to a slight heating of the sample.Indeed, an insig-nicant heating by a few degrees (3-5 K) can noticeably modify EPR signal at 10 K, which is proportional to 1/T (see, e.g.ref. 19  and 20).At the same time, the opposite effect of heating should be present due to the higher population of T 1 state at higher temperatures.Apparently, the former mechanism dominates, causing spectral changes shown in Fig. 9a.Additional perturbation of resonator under laser radiation can also contribute to the observed signal decrease.

Conclusions
In summary, we have synthesized a DPP-derivative bearing two radical groups, i.e. bis(thiophen-2-yl)-2,5-dihydropyrrolo [3,4-c]  pyrrole-1,4-dione terminally capped with nitronyl nitroxide groups (DPP Th -NN 2 ).Notably, we used palladium-catalyzed cross-coupling reaction of the corresponding dibromoderivative with the (nitronyl nitroxide-2ide)(triphenylphosphine)gold complex as a convergent step to synthesize DPP Th -NN 2 diradical.The incorporation of two radicals resulted in interesting spectroscopic, photophysical, and magnetic properties, and the expected unusual excitedstate dynamics.A computational study predicted that the DPP Th -NN 2 diradical has singlet ground state with a small singlet-triplet energy gap DE ST of about 0.2 kcal mol −1 in solution.This value is consistent with strong on the EPR scale exchange interactions observed in the EPR.The inclusion of two NN radicals leads to an increase in the number of excited states both due to excitations involving orbitals localized on NN fragments and due to the exchange interaction of these fragments.Consequently, the density of states increases and, in turn, the energy gaps between them decrease.This effect, combined with the removal of spin prohibition for certain transitions, is expected to signicantly speed up the relaxation process of the excitation initially localized on the DPP Th core.Although we were unable to experimentally detect the lowenergy quintet state, likely due to its brief lifetime or low yield, future studies using ultrafast techniques, such as femtosecond spectroscopy, could potentially reveal its properties.

Materials and instrumentation
All chemicals and reagents were purchased from commercial suppliers and used without further purication.Solvents used for spectroscopic measurements were spectral grade quality.2,5-Dihydro-1,4-dioxo-3,6-dithienylpyrrolo[3,4-c]-pyrrole (2) was synthesized by adapting reported procedure. 21All reactions were monitored by thin-layer chromatography (TLC) carried out on silica gel plates.Preparative separations were performed by column chromatography on silica gel grade 60 (0.040-0.063 mm) from Merck.
1 H and 13 C NMR spectra were recorded on Bruker Avance 300 (300 MHz) spectrometer.The chemical shis were reported in ppm and referenced to the residual solvent peak.s = singlet, d = doublet, t = triplet, m = multiplet, b = broad.Infrared spectroscopy measurements were conceded on Nicolet 730 FTIR spectrometer equipped with an attenuated total reection (ATR) setup.The UV/Vis spectra were recorded at 298 K with a Perki-nElmer Lambda 900 spectrophotometer.Matrix-assisted laser desorption ionization time-of-ight (MALDI-TOF) mass spectra were acquired with a Bruker Reex II MALDI-TOF mass spectrometer, calibrated against a mixture of C 60 /C 70 .The X-ray crystallographic data for the molecules were collected on a STOE IPDS 2T diffractometer using Cu-Ka ImS source.Crystal structure contains two independent molecules of NN 2 -DPP (A and B) both of them are located on an centre of inversion which result in a C i symmetry.The alkyl chains are disordered.One solvent molecule (CH 2 Cl 2 ) complete the unit cell.

Computational details
All quantum chemical calculations were performed for model structures that differ from the structures of the compounds under study by replacing the long n-butyl substituents with methyl groups.This substitution should only slightly affect the electronic and spectral properties of the parent DPP Th and the DPP Th -NN 2 diradical.
To analyze magnetic properties of polycrystalline samples, all quantum chemical calculations were performed for the model geometry of DPP Th -NN 2 diradicals and their pairs obtained from XRD analysis.The parameters of the intramolecular exchange interaction ðH ¼ À2J b S1 b S2 Þ were computed using the accurate ab initio CASSCF 22 and CASSCF/NEVPT2 (ref.23  and 24) procedures.To calculate the J parameters for intermolecular exchange interactions, the spin-unrestricted brokensymmetry (BS) approach 25 at the BS-B3LYP/def2-TZVP level of theory [26][27][28] using the Yamaguchi formula 29 was utilized To rationalize the results of experiments performed in solutions, calculations were carried out using the model geometry optimized in toluene solution at the UB3LYP/def2-TZVP and UM06-2X/def2-TZVP levels of theory; 30 the solvent was taken into account according to the CPCM model. 31he energies of electronic transitions and their oscillator strengths in the electronic absorption spectra of DPP Th and DPP Th -NN 2 were calculated using the time-dependent DFT 32 at the TD-B3LYP/def2-TZVP level.In the case of DPP Th -NN 2 , the calculations were performed for the high-spin triplet state of diradical.The zero-eld splitting parameters (ZFS, D and E/D) for the lowest-energy triplet state of the DPP Th -NN 2 diradical was calculated at the RO-BP86/def2-TZVP level, 33,34 while the g-tensor and hyperne splitting tensors at the B1LYP/def2-TZVP level. 35he energies of low-lying states (2 singlets, 3 triplets and 1 quintet) were also calculated at the CASSCF(14,13)/NEVPT2/ def2-TZVP level.Moreover, the splittings of the triplet and quintet multiplets (ZFS) due to the dipolar spin-spin interaction and spin-orbit coupling (SOC) were evaluated using CASSCF (10,10) wavefunctions and quasi-degenerate perturbation theory (QDPT), 36 as realized in the CASSCF and MRCI modules of the ORCA 5.0.5 soware package. 37The contribution of SOC was calculated using the mean-eld formalism (SOMF(1X)) 38 and found to be negligible.To the best of our knowledge, this work is the rst to provide high-level calculations of the low-energy spectrum of a dye substituted with two radical fragments.
The same soware package was used for all other calculations.The molecular orbitals were visualized using the Chem-cra soware. 39nthetic methods and characterization 3,6-Di(thiophen-2-yl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4dione (2). 21To a 250 mL two-neck round bottom ask equipped with magnetic stirrer, potassium t-butoxide (6.5 g, 57.9 mmol), 2-thiophenecarbonitrile (1) (5.0 g, 45.9 mmol) and tert-amyl alcohol (30 mL) were added and the mixture was heated to 100 °C under a nitrogen atmosphere.At this temperature, a solution of di-ethyl succinate (3.85 g, 22.1 mmol) and tert-amyl alcohol (5 mL) was added to the reaction mixture over 1 h using a dropping funnel.The reaction mixture was stirred at 100 °C for 20 h, and then cooled to room temperature, neutralized with glacial acetic acid (35 mL), and gently reux temperature for 1 h.The resulting pigment suspension was suspended in water-methanol mixture (1 : 1, 50 mL) and ltered to get pigment cake, which was washed with water-methanol mixture until no color found in washings.The crude compound was dried at 100 °C in vacuo, and obtained 4.0 g of compound 2. The product was used in the next step without further purication.

Single-crystal XRD analysis
Single crystal X-ray diffraction data for DPP Th -NN 2 were collected at 193 K on a STOE IPDS 2T diffractometer with Cu-K a ImS mirror system (Table 2).The structure was solved using direct methods, expanded with Fourier techniques and rened with the SHELXT soware package.All non-hydrogen atoms were rened anisotropically.Hydrogen atoms were included in the structure factor calculation on geometrically idealized positions.Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no.CCDC 1908216.†

Photoinduced EPR experiments
Pulse EPR experiments were conducted at Bruker Elexsys E580 spectrometer at the Center of Collective Use ''Mass spectrometric investigations'' SB RAS at X-band (9 GHz).Spectrometer is equipped with ow helium cryostat (Oxford Instruments) and thermocontroller Lakeshore and allows measurements at T = 4-300 K.In order to measure echo-detected (ED) EPR spectra and transverse (phase) relaxation times (T 2 ) we used standard twopulse sequence with the pulse lengths being 10 and 20 ns for p/2 and p pulses, respectively.In case of T 2 measurements, interpulse delay was incremented.Time-Resolved (TR) EPR experiments were performed using homemade spectrometer based on commercial Bruker EMX microwave bridge.In all cases the studied compounds were dissolved in toluene in concentrations ca.0.5 mM and placed into quartz sample tubes with outer diameter of 2.8 mm.The samples were degassed by a few freeze-pump-thaw cycles and then sealed off in vacuo.EPR studies were performed for two samples: DPP Th -NN 2 biradical and its photosensitive moiety DPP Th free of radical fragments.
Laser irradiation was provided by Nd:YaG LOTIS-TII system at 532 nm, power ∼ 20 mJ per pulse, 10 Hz repetition rate.In case of pulse EPR detection laser was synchronized with EPR spectrometer; laser pulse proceeded the rst microwave pulse by 700 ns.In TR EPR experiments the detection was also synchronous with laser pulsing; microwave absorption was detected starting from 100 ns till 9 ms aer the laser pulse.

Fig. 2
Fig. 2 Structures of crystallographically independent molecules A and B (with solvate CH 2 Cl 2 ) in crystals of the DPP Th -NN 2 diradical.

Fig. 3
Fig. 3 (a) Solid state molecular packing of DPP Th -NN 2 with p-p-interactions; (b) side view of DPP Th -NN 2 with p-p-distances between centroids of the DPP Th moieties.

Fig. 4
Fig. 4 (a) Short C-H/O NO contacts and the shortest O/O distances between neighbouring DPP Th -NN 2 molecules in a stack and (b) between DPP Th -NN 2 molecules belonging to different stacks.

Fig. 5
Fig. 5 Normalized electronic absorption spectra of the DPP Th precursor (black curve) and DPP Th -NN 2 diradical (red curve) measured in toluene at room temperature, as well as the calculated positions and oscillator strengths (f, right axis) of electronic transitions depicted as black bars for DPP Th and red bars for DPP Th -NN 2 .

Fig. 7
Fig. 7 Energy diagram of the six lowest energy states of the DPP Th -NN 2 diradical.The energies were calculated at the CASSCF(14,13)/ NEVPT2/def2-TZVP level for the optimized geometry.The splitting of the lowest excited quintet state is calculated at the CAS(10,10)/QDPT level.
DPP Th --−0.042(−44.9)c 0.20 c , where b SDPP Th is the spin of the DPP Th core triplet state, and c S1 and c S2 correspond to the NN radical fragments.b ZFS parameters estimated at the CASSCF(10,10)/QDPT level.c ZFS parameters estimated at the RO-B3LYP level.© 2024 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 6178-6189 | 6183 Paper RSC Advances

Fig. 8
Fig. 8 Experimental (black) and simulated (red) continuous-wave (CW) X-band EPR spectra of DPP Th -NN 2 in toluene at room temperature (a) and in frozen toluene at 135 K (b) (C z 0.1 mM).

Fig. 9
Fig. 9 Echo-detected EPR spectra (a) and phase relaxation kinetics (two-pulse echo decay vs. interpulse delay) (b) the DPP Th -NN 2 diradical in toluene glass at T = 10 K in the presence/absence of photoexcitation.Kinetic curves are normalized to the maximum.